The present invention relates to testing wireless devices that employ multiple-input-multiple-output (MIMO) signal technology.
Many advanced wireless devices employ MIMO technology as a way to increase range or increase data throughput. In MIMO, a device will typically have multiple transmitters and receivers. If the same data is sent using multiple transmitters via the same channel, and received via multiple receivers, the effective operating range can be increased compared to a device sending the data using a single transmitter and single receiver.
If a data to be sent in a single stream is divided up into distinct and separate streams and sent simultaneously by multiple transmitters using multiple antennas via the same channel; and that data is received by multiple receivers using multiple receive antennas; it can be signal processed and decoded into the distinct and separate streams, then recombined, effectively increasing the throughput by having sent two or more simultaneous streams of data using the same channel.
Devices that use MIMO technology for increased range or throughput are tested against a wireless standard's prescribed test for such characteristics as sensitivity, error rates, and modulation quality (e.g. error vector magnitude or EVM). One testing innovation involves simultaneous transmission of data-packet signals by multiple transmitters and the combining of the distinct data-packet signals into a composite data-packet signal. If that composite signal can be evaluated much as a single signal produced by a single transmitter, the test time would be comparable, but only requiring a single VSA to perform the measurements thus lowering the cost. Composite testing is known in the art (U.S. Pat. No. 7,948,254—Digital Communications Test System for Multiple Input, Multiple Output (MIMO) Systems; and U.S. Pat. No. 7,706,250 Apparatus and Method for Simultaneous Test of Multiple Orthogonal Frequency Devision Multiplexed Transmitters with Single Vector Signal Analyzer), but some additional requirements are needed to make this technology work. The data pattern must be known and only direct-mapped signals are supported if power measurement is to be included.
However, MIMO devices having three or more transmitters and receivers (e.g. 3×3 or 4×4 MIMO device) makes using this composite-signal approach less attractive because direct mapping is required to analyze the power. A 2×2 MIMO system derives no real benefit in utilizing spatial mapping, but when you have fewer streams than transmitters (e.g. two streams and three transmitters), the device will normally force the use of spatially mapped signals, as it enables equal transmit power for each stream while utilizing all available transmitters. While it is still possible to measure composite EVM for spatially mapped signals , one cannot measure individual transmitter power when streams are spatially separated. Naturally, one can use direct mapping using the highest data rates during test in a cabled environment (e.g. where the individual transmitters are connected through a combiner or switch to a tester's VSA), but many data rates and corresponding power levels will not be supported if using composite measurement. More importantly, with over-the-air (OTA) testing, one cannot maintain direct mapping as the signals will combine in the air. Even 2×2 devices will have problems in OTA test as direct mapping cannot be maintained.
Consequently, in testing MIMO devices with 3×3 configurations or higher, in the absence of practical and comprehensive composite-signal testing, one is faced with using a multi-VSA solution in order to obtain the needed analysis capability. In other words, one would have, say, three expensive high-performance VSAs to test a device having three transmitters. Similarly, for conducting RX tests on a device having three receivers, where discrete signals sent to each RX antenna are tested for true MIMO signal characteristics, it would also require three expensive high-performance VSGs.
Accordingly, it would be desirable to overcome this limitation by combining a composite EVM testing approach with a low-cost transceiver add on to address the limitations of composite EVM and three-receiver tests addressed above.